Transceivers are often arranged to transmit and receive signals simultaneously. The transmitter and receiver of a transceiver arranged in such a manner generally operate at different frequencies, but in many circumstances it is desirable that those frequencies are as close together as possible. A problem with such transceivers is that the transmitted signal leaks into the receiver whilst the receiver is attempting to receive another signal.
EP441500, the system of which is shown schematically in FIG. 1, describes a system designed to address this problem. A transmitter 1 and receiver 2 are coupled to a common antenna 3 via an antenna duplex filter 4 comprising two bandpass filters 5, 6 connected respectively to the transmitter and the receiver. The receiver comprises an amplifier 7 connected to a notch filter 8 which is connected to the remainder of the receiver 9. The notch filter attenuates signals in a specific frequency range to low levels. This range is chosen to centre on the transmit frequency.
A further problem associated with radio receivers is that of insertion loss. Insertion loss is the decrease in transmitted signal power resulting from the insertion of a device in a transmission line. EP441500 teaches that the insertion loss is less critical if the notch filter 8 is positioned directly at the output of the amplifier 7 so that the amplified signals can be directly filtered. However, when receiving a signal onto which the leaked transmitted signal is superimposed, the notch filter of EP441500 must cope with a dynamic range of signal amplitude. This is because the leaked transmitted signal usually has a much larger signal amplitude than the received signal since it is generated in the transceiver whereas the received signal is generated external to the transceiver and has suffered considerable attenuation by the time it reaches the receiver. Filters designed to cope with such dynamic signal amplitude are undesirable because they are bulky and expensive.
There is thus a need for an improved receiver design which is smaller and cheaper to make, whilst having the advantages of accurately isolating a wanted signal from an interfering signal and protecting received signals from insertion loss.